1. Field of the Invention
This invention relates generally to ceramic materials. More particularly, this invention relates to methods for identifying ceramic materials suitable for use as thermal barrier coatings, and novel ceramic materials discovered by such methods, comprising yttrium aluminum garnet or other ceramics with the garnet structure and alloys thereof. These ceramic materials exhibit low thermal conductivity, high thermal expansion, phase stability to high temperatures, and potentially improved durability.
2. Brief Description of the Prior Art
Thermal barrier coatings (TBCs) are important for the insulation and protection of combustors and turbine vanes used in gas-turbine engines, for example in jet aircraft and power generators. Plasma-sprayed TBCs provide metal temperature reductions of as much as 150.degree. C. (300.degree. F.). The introduction of TBCs deposited by electron beam physical vapor deposition processes over the past five years has provided a major improvement in the durability of prior art TBCs. However, to meet aggressive gas turbine engine performance goals for efficiency and durability, TBCs will be required on turbine airfoils and other hot section components. For the successful application of TBCs to these advanced gas turbine engines (which have turbine inlet temperatures of greater than 1425.degree. C.), TBCs of greatly improve durability and performance will be required.
TBCs in production today comprise a ceramic-metal composite having a metal substrate, a metallic bond coat (on the order of 50-125 .mu.m thick) disposed on the metal substrate, and a protective ceramic coating (on the order of 125-500 .mu.m thick) disposed on the metallic bond coat. The metal substrate is commonly a superalloy, such as MAR-M509 Rene N5; PWA 1480, PWA 1484, and the like. The metallic bond coat commonly consists of a MCrAlY overlay coating or a platinum aluminide (PtAl) diffusion coating. During heat treatment and service, these metallic bond coats form a thin, alumina (Al.sub.2 O.sub.3) film between the bond coat and the protective ceramic coating.
Presently, the protective ceramic coating is made of polycrystalline zirconia (ZrO.sub.2)-based ceramics, including yttria-stabilized zirconia (YSZ) alloy ceramics. Unfortunately, zirconia-based ceramics fail prematurely by spallation during service, thereby exposing the underlying metal to hot gases. This failure is of particular concern in the context of future gas-turbine engines designed for higher operating temperatures and long lives. A number of factors contribute to TBC failure, one of the most important being oxidation of the metallic bond coat. The metallic bond coat is an oxidation-resistant bonding layer present between the metal substrate and the zirconia coating. Oxidation is due to the gas-turbine engine itself providing an environmental oxygen source. The oxygen is readily transported to the bond coat via microscopic defects, such as microcracks and pores, in the zirconia-based ceramic coating. Such defects are intentionally manufactured into the coatings, in order to improve thermal insulation by interrupting heat flow, and in order to relieve thermal-expansion-mismatch strain between the metal substrate and the ceramic coating.
Even where these defects are absent or non-percolating in nature, oxygen is nonetheless readily transported by diffusion through the zirconia lattice. The fast diffusion of oxygen is due to the high concentrations of oxygen vacancies present in the zirconia lattice. Notably, this high concentration of oxygen vacancies also contributes to the low thermal conductivity of zirconia. The usefulness of zirconia-based TBCs thus appears to be fundamentally limited. Accordingly, there remains a need for effective TBCs of high durability in oxygenated, very high temperature environments, and systematized methods for their discovery.